Charged particle beams, laser beams, and neutral particle beams are used in a variety of microfabrication applications, such as fabrication of semiconductor circuitry and microelectromechanical assemblies. The term “microfabrication” is used to include creating and altering structures having dimensions of tens of microns or less, including nanofabrication processes. “Processing” a sample refers to the microfabrication of structures on that sample. In charged particle beam systems, such as electron microscopes or focused ion beam (FIB) systems, a source generates charged particles which are then focused by an optical column into a beam and directed onto the surface of a target to be imaged and/or processed. In the column, this beam may be deflected to move it around on the target surface. As smaller and smaller structures are fabricated, it is necessary to direct the beam more precisely.
One aspect of semiconductor manufacturing that requires accurate beam positioning is the extraction of thin samples for transmission electron microscopy. Such samples are used for monitoring the semiconductor fabrication process. In some applications known as slice and view applications a “slice” or thin, vertical sample, referred to as a lamella, is milled out of the surface of a work piece. The lamella is extracted to leave an exposed cross-sectional surface to be imaged, for example, by a transmission electron microscope (“TEM”). In order to obtain a cross-sectional surface that is as flat and vertical as possible a clean, fine cut from the beam is required.
Typically, a beam controller directs the beam to specified coordinates or along a specified path using a beam controller coordinate system in response to a stored program, pattern generator, or operator instructions to perform a specific process. Ideally, the beam converges in the plane of the work piece or target specimen. However, if the system is not calibrated, the beam may converge before or after the sample plane causing the beam to be unfocussed. Also, the beam may exhibit stigmatic effects. Moreover, there may be rotational misalignment between the axis of the specimen and the scan axis of the beam. Or there may exist a non-orthogonal relationship between the beam axes. Further, the scan gain may be different in each of the orthogonal scan directions so that in one direction the image appears “stretched”. The scanned beam system must therefore be calibrated to eliminate or at least minimize these errors. To overcome these problems a beam system will typically provide control elements to achieve calibration. For example, an electrostatic lens system may be provided to cause the beam to converge at the correct focal point and a stigmator may be provided to adjust for stigmation effects.
For precision processing in which a charged particle beam is used to mill material from the surface of a target the milling rate is roughly proportional to the beam current. For example, higher beam currents are preferred when it is desired to quickly remove material. However, beams using higher current are less precise and typically result in a damaged or undesirable sample. Therefore, lower beam currents have been used for more precision processing applications. A beam having a lower beam current, that is, fewer charged particles per second, can typically be focused to a smaller size than a beam having a higher current. For example, a small beam with a lower current is more precise and results in less unwanted damage to the sample. See, for example, U.S. Pat. No. 6,949,756, entitled “Shaped And Low Density Focused Ion Beams” to Gerlach et al., assigned to the assignee of the present invention, which is hereby incorporated by reference. However, use of lower current beams reduces the rate of material removal and results in a longer processing time.
It is increasingly more desirable to decrease processing time. Non-round or “shaped” beams have been developed in order to increase milling speed. Shaped beams can be generated having a straight edge for cutting away material, and at the same time, having their beam spot shapes of a size with enough beam current for quickly removing material. See, for example, U.S. Pat. No. 6,977,386, entitled “Angular Aperture Shaped Beam System And Method” to Gerlach et al., assigned to the assignee of the present invention, which is hereby incorporated by reference.
In some systems the beam is shaped by an aperture plate positioned within the optical column having one or more selectable beam-defining apertures (BDAs) through which the beam passes. The BDAs are typically holes in a metal strip, which are intended to block the off-axis charged particles and to pass the ones used to form the beam. There are typically several BDAs, or holes, in a metal strip, and the apertures can be switched depending on the application by moving the strip so that a hole of a different size and/or shape is positioned in the path of the beam. For example, a beam having a desired geometrically shaped spot is formed by a shaping aperture typically disposed between one or more lenses in a charged particle column. To achieve calibration, control elements may include a two-lens focusing column in which the first lens forms an image of the beam at or near the plane of the second lens and the second lens forms an image of the shaping aperture on the target plane. The lenses and other “optical” elements (i.e., a stigmator) in the beam column may use electrostatic or magnetic fields to align the beam along the optical axis and focus the beam on the target plane.
It is important for the beam to be accurately focused and compensated for aberrations. Typically, a beam is focused and aligned using known values for the optical elements such as the lenses and stigmator from a table of values that have been input and stored in a program or through operator instructions based on desired current. For example, FIGS. 1A and 1B show spot burn arrays for a 130 pA round beam. FIG. 1A shows a spot burn array 100 where the second lens focus was varied in a serpentine fashion from top to bottom and left to right. The beam was optimized using an image sharpness routine based on auto focus so that the center spot 102 is near optimal. FIG. 1B shows a spot burn array 104 where the stigmation was varied along X and Y directions. The beam was optimized using an image sharpness routine based on stigmation so that the center spot 106 is near optimal.
However, it is difficult to predict the optimal focus and stigmator settings for shaped beams as illustrated in FIGS. 2A and 2B, which show spot burn arrays for a 420 pA shaped beam formed with an elliptical aperture. FIG. 2A shows a spot burn array 108 where the second lens focus was varied in a serpentine fashion from top to bottom and from left to right. The beam was optimized using an image sharpness routine based on auto focus on its corresponding round beam so that the center spot 110 is near optimal. FIG. 2B shows a spot burn array 112 where the stigmation was varied along X and Y directions. The beam was optimized using an image sharpness routine based on stigmation on its corresponding round beam so that the center spot 114 is near optimal. However, as can be seen in FIGS. 2A and 2B the optimal spots 110, 114 are not elliptical but are more flat. This is because the auto focus/stigmation routines applied to elliptical beams (or other shaped beams) will typically seek out the roundest beam. However, the roundest-shaped elliptical beam is sub-optimal for milling.
What is needed is a method and system for optimizing a shaped beam having a sharp edge to achieve clean and fine milling operations while having a high current for rapid processing.